This invention relates generally to a flowmeter of the rotameter type having a variable-area tube, and more particularly to a rotameter in which the meter tube is protectively encapsulated in a plastic bar.
In the usual type of rotameter, a weighted plummet or float contained in an upright tapered tube is raised to a position of equilibrium between the downward force of the float and the upward force of the fluid flowing past the float through the annular orifice surrounding the float. The term "rotameter" was derived from the fact that plummets originally had slots to impart a rotational force thereto for the purpose of centering and stabilizing the float. The present trend, however, is toward non-rotating floats.
In one well-known commercial form of rotameter, such as that disclosed in U.S. Pat. No. 3,342,068, in lieu of a tapered tube, the flowmeter is provided with a tube having a tapered bore affording a variable cross-sectional area. The float disposed in the bore assumes a vertical position depending on the rate of fluid flow, which may be of liquid or gas. The vertical position of the float is indicated along a calibrated scale on the front of the tube.
Because the meter tube is made of thin-walled glass and is relatively fragile, in existing types of variable-area flowmeters, the tube is supported by end fittings mounted within a case adapted to protect the tube against stresses as well as to effect tight sealing thereof. In the typical arrangement, the end fittings for the tube are attached to the case at opposing positions therein, and inlet and outlet adapters are provided to facilitate insertion of the tube in the end fittings.
As operating pressure is increased in the flowmeter tube, the ends of the glass tube are subjected to a substantial pressure from the end fittings. Since the end fittings are confined within the case, the pressure on the end fittings is applied to the case and because of the pressure developed at the ends of the tube, there is a tendency for the case to warp. As the case distorts, it transmits a bending moment to the tube.
On a long, slender tube, the resultant cross-bending gives rise to a distinctly visible bow. In a shorter tube, the amount of bowing is less evident to the naked eye, but it is reflected in lowered strength values for the tube. It has been found that the bowing phenomenon experienced in conventional flowmeter tube arrangements will in some instances result in breakage of the tube. Thus the very meter case intended to protect the tube is responsible for damage thereto. Moreover, existing case or frame arrangements for variable-area flowmeters add materially to the cost of the meter.
The pressure rating of a variable-area metering tube depends on the tensile strength characteristics of the glass used to form the tube, as well as on its wall thickness. However, the maximum pressure rating of a given meter affords no assurance that breakage will not occur at lower pressures.
An exposed glass tube of good inherent strength may be weakened by scratches on the surface thereof, and should the tube shatter in the course of the operation, serious injuries may be inflicted on personnel in the vicinity thereof. Also the manner of installation may be defective, and give rise to stresses resulting in tube fracture. While it has sometimes been the practice to provide safety enclosures for the variable-area glass tube meters, this adds materially to the cost of installation and also interferes to some extent with the readability of the meter.
In order to overcome the drawbacks encountered with thin-walled variable-area glass flowmeter tubes and to avoid the need for end fittings to support the tube, it is known to make use of a relatively thick molded glass block which is drilled, polished and otherwise processed to define a longitudinally-extending internal passage which is tapered to provide suitable variable area characteristics. The ends of the passage communicate with lateral bores adapted to receive retainer elements which serve to attach the block to a meter frame and to couple the upper and lower ends of the passage to inlet and outlet pipe couplers.
One such glass meter bar arrangement is disclosed in Instruction Bulletin 10 A 1350, published February, 1974 by Fischer & Porter Co. of Warminster, Pa. The difficulty with this arrangement is that the fabricating costs of the glass block are high, and while a glass block is stronger than the conventional thin-walled tube, it still has the inherent limitations of glass and is subject to breakage. As indicated in the Bulletin, the strains transmitted to the glass when tightening process pipes or when replacing the retainers may result in damage thereto.
An alternative approach to overcoming the limitations of a thin-walled glass meter tube is that disclosed in U.S. Pat. No. 3,768,309 wherein the tube and end fittings therefor are encapsulated in a clear plastic jacket. The problem with such an arrangement is that it is very difficult, as a practical matter, to encapsulate both the tube and the end fittings. Moreover the plastic tube and end fittings assembly is limited to meter structures adapted to cooperate with the particular fittings integrated with the glass tube.